This invention relates to devices and methods for measuring odor, or odorous components in a sample gas, by means of an odor sensor which is a kind of gas sensor. Such odor measuring devices may be found useful in many fields of application such as the quality control of foods and perfumes, the quantitative analysis of public nuisance due to odor, the fire prevention by detecting a burning odor, and even in the police work such as in the tracing and identification of a suspected person or discovery of illegal chemicals.
An odor sensor is adapted to electrically or optically measure a change which takes place in the sensor when an odorous component in air or in a sample which is supplied thereto becomes attached to the sensor""s odor-sensitive surface. Odor sensors using semiconducting oxides and electrically conductive polymers have been known.
FIG. 3 shows an example of a prior art odor measuring device using such an odor sensor. When a pump 25 is activated, purified air deprived of floating dust particles and water components by a dust-removing and dehumidifying unit 40 is introduced into a sample container 41 storing therein a sample liquid which contains an odorous component, and a sample gas containing the odorous component is supplied from the sample container 41. As this sample gas containing the odorous component is introduced into a flow cell 27 which contains an odor sensor 26 therein, the odorous component is adsorbed to an odor-sensitive film of the odor sensor 26, causing a change in the resistance between electrodes of the odor sensor 26. There is a measuring unit 28 which serves to measure this change, and the sample gas leaving the flow cell 27 is discharged through the pump 25. The use of purified air as the carrier gas for the odorous component serves not only to prevent corrosion of the odor-sensitive film and the electrodes but also to reduce erroneous response of the odor-sensitive film due to unwanted substances which may also become adsorbed thereto.
Odor sensors using an odor-sensitive film comprising an electrically conductive polymer are coming to be used recently, but their sensitivity is affected as the odor-sensitive film become oxidized by the oxygen gas in air. If such an odor-sensor is used in an odor measuring device as described above, the odor-sensitive film is gradually oxidized due to the oxygen component in the sample gas, making it difficult with the passage of time to obtain results with high repeatability. Since the oxidation of a conductive polymer film is accelerated particularly in an environment with high humidity, the deterioration of the odor-sensitive film becomes a severe problem when the measurement is carried out at a high temperature or the odor sensor is operated at a high temperature in order to remove the odorous component which has become adsorbed. In other words, the useful lifetime of the odor-sensitive film is short, and it must be replaced frequently. This naturally adds not only to the cost of operation but also to the time required for the measurement.
Another problem to be considered is that a process for gas condensation by thermal desorption is frequently carried out, when the odorous component is at a relatively low concentration in a sample gas, in order to increase the concentration of the target component to be measured. By the thermal desorption method, the sample gas is passed through a condenser tube filled with a material which mainly adsorbs the target component. After the target component has been sufficiently adsorbed thereto, a carrier gas is passed through this condenser tube while its temperature is raised suddenly such that the once adsorbed target component is desorbed quickly and is introduced into the sensor at a higher concentration.
In the case of a sensor using a semiconducting oxide such as tin oxide as the odor-sensitive film, what is actually measured is a change in the conductivity of the semiconducting oxide due to the oxidation-reduction reaction between oxygen and a reductive gas adsorbed to the semiconducting oxide. Thus, the carrier gas for transporting the target component to be measured must be one which contains oxygen, such as air. In other words, the agent which fills the condenser tube must be of a kind which does not degenerate although it is heated to a high temperature in the presence of oxygen. In general, different agents must fill the condenser tube for the adsorption of different target components to be measured. Thus, there is usually only a limited number of agents that can be used. For example, ordinarily used agents comprising carbon will deteriorate significantly if heated in the presence of oxygen and hence cannot be used in the condenser tube of an odor measuring device.
It is therefore an object of this invention to provide an improved odor measuring device capable of preventing or reducing the deterioration of its sensor due to oxidation.
It is another object of this invention to provide such an odor measuring device for which agents of many different kinds can be used when an odor sensor such as comprising a semiconducting oxide requiring oxygen for the detection of the odorous component and a condenser device using the thermal desorption method are combined.
It is additionally an object of this invention to provide methods of measuring odor, or components with odor in a sample gas, by using such devices.
An odor measuring device according to this invention, with which the first of the above objects can be accomplished, may be characterized as comprising a collector tube containing an adsorbent which adsorbs sample components with odor at normal temperatures and desorbs these sample components when heated. Flow routes for a sample gas containing sample components to be detected and an inactive gas are connected through switchable valves to this collector tube. Initially, a sample gas containing sample components with odor is introduced into the collector tube at normal temperature such that the sample components to be detected are adsorbed to the adsorbent. Thereafter, the valves are switched and the gas flow routes are changed such that the inactive gas is introduced into the collector tube while its temperature is increased by a heater, causing the sample components to be desorbed into the inactive gas and to be transported thereby into an odor sensor.
The second of the above objects can be accomplished by further providing means for mixing an oxygen-containing gas such as air into the inactive gas after the latter has passed through the collector tube. With a device thus structured, the carrier gas which transports the sample components to the odor sensor is mostly inactive but also contains some oxygen. Thus, odor sensors with semiconducting oxides which require oxygen for the detection may be used but since the oxygen-containing gas is mixed to the inactive gas after the latter has passed the collector tube, the adsorbent in the collector tube does not come into contact with the oxygen-containing gas and hence is not oxidized.
When sensors of both types requiring and not requiring the existence of oxygen for detection, such as sensors with electrically conductive polymers and semiconducting oxides, are used together, the odor detecting means may be divided into a first part consisting of sensors not requiring oxygen for detection and a second part consisting of those requiring oxygen for detection and the oxygen-containing gas may be mixed to the inactive gas after the latter has passed through the first part. With the odor detecting means thus structured, the oxygen-containing gas which is mixed in does not come into contact with the sensors of the first part such as those using conducting polymers and hence deterioration of such polymers by oxidation can be prevented.